ABSTRACT Childhood obesity is a top public health concern and its prevalence continues to increase in disadvantaged populations. Evidence suggests that obesity is associated with neighborhood built and social environment characteristics. These include walkability, exercise opportunities, green space, food outlets and social cohesion. But it is not clear whether these characteristics themselves cause obesity. Establishing causality is important to inform public health efforts to reduce health disparities. We propose to address this gap by studying a natural experiment in a low-income urban community. The city of Los Angeles is redeveloping Jordan Downs (JD), an obsolete 700-unit public housing project. Over 5 years, the redevelopment will build?(1) 700 new units for existing residents, (2) 700 units for new mixed-income housing residents that will alter the social environment, (3) a new community center with a gymnasium, (4) retail space that will offer healthy eating opportunities, (5) green space that will increase opportunities for physical activity, and (6) additional built environment enhancements that will promote physical activity and healthy eating (e.g. walking and bike lanes, street lighting). In Dec 2017, we will begin a study to examine the effects of these improvements on adult residents' BMI and obesity. It is important to extend this work to children because childhood obesity is associated with poor life-long health outcomes. Our goal is to study the effect of the JD redevelopment on children's BMI, overweight and obesity using a similar research design as for the Adult Study. In Aim 1, we will compare changes in these outcomes for JD children and a control group (children from 2 similar public housing projects) for 5 years. In Aim 2, we will disentangle the effects of three major components of the redevelopment: built environment, social environment and new housing. We will do this by taking advantage of the fact that the redevelopment will occur in several phases. In Aim 3, we will assess why, or why didn't, the redevelopment affect children's BMI outcomes. To do this, we will track diet, activity, and potential mediators (utilization of new spaces, parent modeling, perceived safety, home environment). Finally, in Aim 4 we will explore the moderating effects of child sex, race-ethnicity and age. Our data collection from children will include in-person surveys, body measurements, dietary recalls, and accelerometry. In addition, a wealth of data about the parent will be available from the Adult Study. Our study is time-sensitive because the first phase of redevelopment will be completed by fall of 2018. Thus, recruitment and baseline data collection from children must be completed along with the Adults before Oct 2018.